The present invention relates to electrophotographic exposure methods and arrangements. More particularly, the present invention relates to a memory arrangement for exposure data used in multiple exposures with gray levels and a method for processing the exposures carried out using the memory arrangement.
An electrophotographic arrangement such as a copying machine or a laser printer carries out printing by electrostatically attracting toner onto a paper sheet. FIG. 1 is a schematic view showing a general electrophotographic arrangement. The arrangement shown in FIG. 1 includes a photoreceptor drum 2, an image exposure light source 4, a toner cartridge 6 and a developer roll 8, producing a toner image which is transferred onto a paper sheet 10. The electrophotographic arrangement also includes additional optical and mechanical elements which are omitted in FIG. 1 for clarity.
In the arrangement shown in FIG. 1, toner is attracted to the photoreceptor drum 2 by an electrostatic force and is then transferred onto the paper sheet 10 to be printed. The photoreceptor drum 2 has a surface layer consisting of a photosensitive material which generates electron/hole pairs under illumination of light of a specified wavelength. During a printing operation, the photoreceptor drum 2 is rotated counterclockwise as shown in FIG. 1. As the photoreceptor drum 2 rotates, operations of charging, exposure, development, transfer, fusing, cleaning and erasure are sequentially carried out. These operations will be described with reference to the positions a, b, c and d on the photoreceptor drum 2 in FIG. 1.
First, at the position a, electrostatic charges are uniformly applied to a surface of the photoreceptor drum 2 by a high voltage. At the position b, a light beam from the exposure light source 4 illuminates the electrostatic charge in the regions on the surface of the photoreceptor drum 2 according to the image to the printed. Since the light from the exposure light source 4 eliminates the electrostatic charges which are illuminated, a latent image is formed, and thus the charge distribution is no longer uniform but corresponds to the image to be printed. At the position c, toner is attracted from the sleeve 8 in the toner cartridge 6 to the surface of the photoreceptor drum 2 at the locations having the electrostatic charges. Finally, at the position d, the toner on the surface of the photoreceptor drum 2 is held against the paper sheet 10 as it passes and is attracted to corresponding locations on the paper sheet 10. The resulting toner image on the paper sheet 10 is fixed by fusing, thereby completing the printing operation. For a subsequent printing operation, the toner remaining on the photoreceptor drum 2 is removed by cleaning and erasure.
As described above, the light beam from the exposure light source 4 changes the electrostatic charges on the surface of the photoreceptor drum 2 according to the image to be printed. Two types of exposure light sources for this purpose are conventional. One type is a light emitting diode (LED) printing head (LPH) and the other type is a laser scanning printing head. FIG. 2 is a schematic view showing an exposure process using a laser scanning head. As shown, a charged surface 2a of a photoreceptor drum 2 is exposed by a laser scanning head 4a. Since such laser scanning heads are large and expensive, in general, only a single laser beam is used for scanning. In FIG. 2, all the pixels in the image to be printed are scanned sequentially by the laser scanning head 4a. That is, the exposure by the laser scanning head is conducted pixel by pixel. On the other hand, FIG. 3 is a schematic view showing an exposure process using an LPH. As shown, an elongated LPH 4b comprises a plurality of LEDs each corresponding to a pixel in a scanning line on a surface 2a of a photoreceptor drum 2, and thus the LPH 4b corresponds to a complete scanning line. For a resolution of 300 dpi (dots per inch) using a paper sheet of letter size with a width of 8.5 inches, the LPH contains about 2550 LEDs. The exposure by the LPH is conducted line by line in contrast to the pixel-by-pixel exposure with the laser scanning head. This difference influences the exposure processing of images containing a plurality of gray levels.
Different gray levels in an image can be represented by various number combinations. For example, four gray levels can be represented by numerals 0, 1, 2 and 3, or binary bits 00, 01, 10 and 11. In gray level exposure, the charges remaining on a photoreceptor drum after exposure are not uniform due to the different exposure levels so that the amount of toner attracted to a sheet to be printed is not uniform. For the laser scanning head, exposure times are controlled by pulse widths to achieve different gray levels. As shown in FIG. 4, pulses of different widths correspond to gray levels xe2x80x9c3xe2x80x9d, xe2x80x9c2xe2x80x9d, xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d. If the maximum pulse width is xe2x80x9cTxe2x80x9d, xe2x80x9cTxe2x80x9d corresponds to gray level xe2x80x9c3xe2x80x9d. The pulse width for gray level xe2x80x9c2xe2x80x9d is smaller than that for gray level xe2x80x9c3xe2x80x9d, and two possible pulse timings are shown in FIG. 4. The pulse width for gray level xe2x80x9c1xe2x80x9d is smaller than that for gray level xe2x80x9c2xe2x80x9d. For the gray level xe2x80x9c0xe2x80x9d there is no exposure. The different exposure times vary the residual charge levels. On the other hand, it is extremely difficult for an LPH to use pulse width modulation to provide different gray levels since it is complicated to control the exposure time of each LED of the LPH individually. Therefore, multiple exposures are used to provide gray levels. For example, two exposures can produce 4 gray levels.
As described above, two or more exposures are employed to achieve different gray levels for an LPH. However, that method has the following disadvantage. During exposure, data for the image to be printed are stored in an exposure data memory. Then the image data are read from the memory and sent to each LED of the LPH. If multiple exposures are necessary for a single image pixel to provide a desired gray level, which means that access to the memory is needed for each exposure, then repeated accesses to the memory is needed for that pixel, thereby lengthening the data access time. In general, the access time needed for an LPH increases with the number of exposures, and it is longer than the access time needed for a laser scanning head using pulse width exposure control.
To overcome the problem resulting from the need for multiple data access, a design using a single bit for each access has been considered. However, it is impossible to overcome the problem in this way with a conventional memory arrangement. For example, for 2-bit gray levels, each time when a numeral representing a gray level of a pixel is to be read, two bits must be read at the same time. Reading of the useless bit cannot be avoided with the conventional memory arrangement. That is, when data at a selected address is being read with the conventional arrangement, it is impossible to further select an odd bit or an even bit.
Accordingly, in view of the foregoing, it is an object of the present invention to provide a memory arrangement for exposure data for multiple exposures which overcomes disadvantages of the prior art.
Another object of the invention is to provide a memory arrangement in which repetition of access to data is decreased during multiple exposures by an LPH, thereby shortening the time needed for the exposures.
A further object of the present invention is to provide a method for processing multiple exposures using a memory arrangement according to the present invention.
These and other objects of the invention are attained by providing a memory arrangement for exposure data used in multiple exposures for storing gray level data in which the memory arrangement has a first memory for storing a high bit portion of the gray level data and reading out the high bit portion in a first exposure and a second memory for storing the low bit portion of the gray level data and reading out the low bit portion in the first exposure. The first memory and the second memory can be controlled by separate chip selection control signals. The memories are further provided with two separate bit part enable signals, respectively to control accesses to a high bit part and a low bit part in each memory unit. During writing, the gray level data are written by first using the high bit part enable signal of the first memory and the low bit part enable signal of the second memory and then using the low bit part enable signal of the first memory and the high bit part enable signal of the second memory, to avoid conflict between data lines. During reading, the high bit part and the low bit part are exchanged before they are read out.